(1) Field of the Invention
The present invention relates to a method for forming a ferroelectric spontaneous polarization reversal in a desired region of a ferroelectric substrate, and more particularly to a method for forming a ferroelectric spontaneous polarization reversal that forms on a substrate having an electrooptic effect that is used for an optical element. In addition, it relates to a method for forming a ferroelectric spontaneous polarization reversal, where a ferroelectric substrate has convexo-concave structure on its surface and at a region including one portion of said convex part. Further, it relates to a method for forming a ferroelectric spontaneous polarization reversal capable of reversing the polarity of a narrow region of a substrate and also, capable of narrowing the interval between ferroelectric spontaneous polarization reversal regions.
(2) Related Art Statement
An optical element such as a wavelength conversion element or an optical modulator is used in optical communication and optical measurement systems.
For example, a wavelength conversion element has periodical ferroelectric spontaneous polarization reversal structures on a substrate with an electrooptic effect such as a ferroelectric LiNbO3, as disclosed in the following patent document 1.
Also, as an example of an optical modulator, the optical modulator which has optical waveguides on a substrate with an electrooptic effect and has a ferroelectric spontaneous polarization reversal structure in one portion of the substrate related to said optical waveguides to suppress chirp generation or to improve extinction ratio of modulation intensity, as disclosed in the following patent document 2, has been proposed.    [Patent Document 1] Japanese Patent Application Publication No. 2000-147584    [Patent Document 2] Japanese Patent Application Publication No. 2003-202530
As a method for forming a ferroelectric spontaneous polarization reversal region on a ferroelectric substrate, Ti thermal diffusion technique, heat treatment after loading SiO2 technique, and a proton exchange and following heat treatment technique can be cited. In addition, a method for forming a ferroelectric spontaneous polarization reversal region by applying an electric field higher than the value of coercive filed (e.g. 20 kV/mm for LiNbO3) is also known.
Specifically, ferroelectric spontaneous polarization reversal by applying an electric field is widely used as a method for forming a ferroelectric spontaneous polarization reversal because it is possible to form a ferroelectric spontaneous polarization reversal region accurately and the method for forming is simple.
As a method for forming a ferroelectric spontaneous polarization reversal by electric field, it has been proposed to apply a voltage 4 through electrodes 2 and 3 which are fabricated on the top and bottom faces of substrate 1 as shown FIG. 1, or to apply voltage 4 through electrodes 6 and 7 where conductive liquid is filled between substrate 1 and each electrode after fabricating insulating patterned mask 5 on the top face of substrate 1 using sealing members 8 and 9 at the same time as fixing said substrate as shown in FIG. 2. In addition, in case of using an insulating material such as an acrylic board instead of electrodes 6 and 7, it is arranged that an electric wire directly contact the conductive liquid for feeding of voltage 4.
By these methods, the ferroelectric spontaneous polarization reversal regions corresponding to the pattern of electrodes 2 and ferroelectric spontaneous polarization reversal regions corresponding to the region where the mask patterns 5 are not formed are formed respectively in FIG. 1 and FIG. 2.
For the methods for forming a ferroelectric spontaneous polarization reversal by electric field as stated above, when the width of a ferroelectric spontaneous polarization reversal region, for example a width L of a ferroelectric spontaneous polarization reversal region 10 formed on substrate 1 in FIG. 3 is less than 20 μm, it is possible to form relatively homogeneous ferroelectric spontaneous polarization reversal regions because a homogeneous voltage is applied all over the region where a ferroelectric spontaneous polarization reversal is to be formed.
On the other hand, when a large ferroelectric spontaneous polarization reversal region having width L of more than 50 μm is formed, it results in inhomogeneous ferroelectric spontaneous polarization reversal condition as a whole because a ferroelectric spontaneous polarization reversal is formed preferentially in the periphery of the region where a ferroelectric spontaneous polarization reversal is expected to be formed. Also, it becomes difficult to form a ferroelectric spontaneous polarization reversal homogeneously because of in-plane variation of wafer thickness and differences in voltage effects that are caused by dispersion of electric resistance of electrodes that apply an electric field. Thus, when a large diameter wafer is used, the differences of ferroelectric spontaneous polarization reversal condition depending on in-plane location of the wafer become prominent.
In the meantime, the following non patent documents 1 to 6 disclose that a process for forming a ferroelectric spontaneous polarization reversal comprises of the nucleation at the concentrated region of an electric field of an electrode edge, the expanding of microdomain in a depth direction so as not to increase electrostatic energy, the movement of domain wall in a transverse direction, and the stabilization of a ferroelectric spontaneous polarization reversal region, and these documents also disclose that the degree of nucleation density is important for homogeneity of a ferroelectric spontaneous polarization reversal.
In other words, it can be easily understood that, when a large region having width L of more than 50 μm for forming a ferroelectric spontaneous polarization reversal, the polarity of the periphery is preferentially reversed, and that it is difficult to form a homogeneous ferroelectric spontaneous polarization reversal when width L is wide because the nucleation density is lower compared with when said width is narrow.
Thus, the non patent documents 1 to 6 propose a method for applying low electric field pulses into a substrate to generate nucleuses that are to be the nucleus of a ferroelectric spontaneous polarization reversal, and subsequently applying a high electric field pulse to extend domain wall from said nucleus to thereby realize a ferroelectric spontaneous polarization reversal.
Moreover, it has been reported that a homogeneous ferroelectric spontaneous polarization reversal region can be obtained by this method.
[Non Patent Document 1]    Sunao KURIMURA et al., “Selective nucleation control for a periodically poled lithium niobate I˜motivation and background˜”, Pre-Texts of the 49th Meeting; The Japan Society of Applied Physics and Related Societies, March 2002
[Non Patent Document 2]    Takeshi AKUTSU et al., “Selective nucleation control for a periodically poled lithium niobate II    ˜Periodical poling by selective nucleation control˜”, Pre-Texts of the 49th Meeting; The Japan Society of Applied Physics and Related Societies, March 2002
[Non Patent Document 3]    Masayuki MARUYAMA et al., “Selective nucleation control for a periodically poled lithium niobate III˜Qualification of nucleation density by particle analysis˜”, Pre-Texts of the 49th Meeting; The Japan Society of Applied Physics and Related Societies, March 2002
[Non Patent Document 4]    Yoshiyuki NOMURA et al., “Selective nucleation control for a periodically poled lithium niobate IV˜Pulse number dependence of nucleation density˜”, Pre-Texts of the 63rd Meeting; The Japan Society of Applied Physics, September 2002
[Non Patent Document 5]    Masayuki MARUYAMA et al., “Selective nucleation control for a periodically poled lithium niobate V˜Nucleation parameters for short period structure˜”, Pre-Texts of the 63rd Meeting; The Japan Society of Applied Physics, September 2002
[Non Patent Document 6]    Masayuki MARUYAMA et al., “Selective nucleation control for a periodically poled lithium niobate VI˜green light SHG with high-aspect-ratio periodicdomains˜”, Pre-Texts of the 50th Meeting; The Japan Society of Applied Physics and Related Societies, March 2003
Also, for an optical element such as an optical modulator, optical element having ridge structure has been put to practical use for the purposes of lowering drive voltage, matching impedance, and expanding bandwidth.
FIG. 7(a) is a perspective view of the optical element having ridge structure and FIG. 7(b) is a cross-sectional view of the substrate along the chain line A in FIG. 7(a). Modulating electrodes or the like are not shown in these figures.
An optical waveguide 112 is formed on a ferroelectric substrate 101 while a ridge 110 is formed on the region including said optical waveguide 112 in FIG. 7. Further, they show a ferroelectric spontaneous polarization reversal region 111 formed on one portion of an optical waveguide 112.
A method for forming a ferroelectric spontaneous polarization reversal in one portion of the ferroelectric substrate and subsequently removing a region where a ridge structure is not formed on the top face of said substrate selectively by mechanical cut or chemical etching as shown in FIG. 8 is a common method for forming a ferroelectric spontaneous polarization reversal in a region having convexo-concave structure such as a ridge structure on the top face of the ferroelectric substrate and including one portion of said convex part as shown in FIG. 7.
Regarding the specific procedures, at first, an electrode 120 having a desired pattern is formed on the top face of a ferroelectric substrate 101 and an electrode 121 is formed on all over the bottom face of said substrate as shown in FIG. 8(a). Then, a high voltage is applied between said electrodes 120 and 121 by a voltage source 122 to form a ferroelectric spontaneous polarization reversal 111 in the region corresponding to the pattern of electrode 120.
After that, electrodes 120 and 121 on the substrate are removed while a mask is formed corresponding to the pattern of a ridge structure formed on the top face of substrate 101. The substrate top face except the mask-covered region is removed chemically by dry etching or wet etching, or mechanically by sandblast or cutting to thereby form ridge structures 123 (see FIG. 8(b)). The concavity and convexity on the substrate surface where an optical waveguide is not formed as shown in the cross-sectional view of FIG. 7(b) are not shown in FIG. 8 to FIG. 14 to facilitate understanding of the description of the present invention.
However, it is difficult to form a desired ridge structure in the chemical removal process because the etching velocity and/or etching direction priority between the ferroelectric spontaneous polarization reversal region and the other region is different, and also, there is a disadvantage that in a mechanical removal process the substrate gets easy to break because the substrate receives much of a shock entirely.
Thus, it is required to form the ridge structure on a ferroelectric substrate firstly, and then, to form a ferroelectric spontaneous polarization reversal in a desired region of said substrate.
FIG. 9 show the method for forming a ferroelectric spontaneous polarization reversal after forming a ridge structure.
At first, optical waveguides 130 are formed on the top face of substrate 101 (see FIG. 9(a)), then, mask members 131 are formed on the region for forming a ridge structure on the top face of the substrate 101. After that, the region where mask members 131 are not formed is removed by etching or the like to thereby form ridge structures 132.
In order to form a ferroelectric spontaneous polarization in ferroelectric substrate 101 which has formed ridge structures 132, at first an electrode 133 is formed on the desired region of the top face of the substrate 101, and at the same time, an electrode 134 is formed all over the bottom face of the substrate 101. Then, a high voltage 135 is applied between both electrodes 133 and 134 to form a ferroelectric spontaneous polarization reversal 136 in the region of the substrate corresponding to the pattern of electrode 133 as shown in FIG. 9(d).
After forming a ferroelectric spontaneous polarization reversal, electrodes 133 and 134 are removed to obtain the substrate having the ridge structures where the polarity of one portion is reversed as shown in FIG. 9(e).
However, the ferroelectric spontaneous polarization reversal method in FIG. 9 has some negative effects such as the substrate becomes easy to break because the electric filed becomes concentrated at the edges 137 of electrode 133 formed on the ridge structure in FIG. 9(d), and further, when forming a ferroelectric spontaneous polarization reversal over the ridge structure and other regions, voltage adjustment in a ferroelectric spontaneous polarization reversal becomes complicated because the ridge and other regions are different in thickness and therefore, are different in strength of the electric field.
On the other hand, the method for using the insulating mask as shown in FIGS. 10 and 11 is cited as a method for forming a ferroelectric spontaneous polarization reversal in a ferroelectric substrate 101 which formed the ridge structure in FIG. 9(c).
In FIG. 10, an insulating mask 140 is formed on the top face of substrate 101 where a ferroelectric spontaneous polarization reversal is not formed. Said substrate is put between electrodes 142 and 143 with sealing members 141 while conductive liquids 145 and 146 are filled between a substrate 101 and each electrode 142 and 143, and a high voltage 144 is applied through the electrodes 142 and 143 like FIG. 2.
This method enables the forming of ferroelectric spontaneous polarization reversal 147 in the region of the substrate where the insulating mask 140 is not formed. The insulating mask is removed away after the ferroelectric spontaneous polarization reversal.
Also, in FIG. 11, an insulating mask 150 is formed on the bottom face of substrate 1 where a ferroelectric spontaneous polarization reversal is not formed. Said substrate is put between electrodes 152 and 153 with sealing members 151 while conductive liquids 155 and 156 are filled between a substrate 101 and each electrode 152 and 153, and a high voltage 154 is applied through the electrodes 152 and 153 like FIG. 2 or FIG. 10.
After that, a ferroelectric spontaneous polarization reversal 157 in the region of the substrate where the insulating mask 150 is not formed is formed, and subsequently the insulating mask 150 is removed.
However, the method for forming a ferroelectric spontaneous polarization reversal using the insulating mask as shown in FIGS. 10 and 11 has a problem that the material for the insulating mask is limited because it is necessary to select the material having electric resistance higher than that of the ferroelectric substrate 101.
Furthermore, although when a resist mask that is widely used as the insulating mask is used here, hard baking treatment is effective in increasing the electric resistance value of a resist mask, it becomes difficult to remove the resist mask after forming a ferroelectric spontaneous polarization reversal. Moreover, the control accuracy of a ferroelectric spontaneous polarization reversal region declines because unintended micro ferroelectric spontaneous polarization reversal regions (micro-domain) are introduced into the substrate entirely due to the heat treatment, and operation troubles or a degradation of characteristics of a device using a ferroelectric spontaneous polarization reversal structure, such as a wavelength conversion element or an optical modulator, could be caused.
In addition, when an electric resistance value of an insulating mask cannot be raised adequately, a ferroelectric spontaneous polarization reversal region sometimes spreads to an unintended region of the substrate as shown by 148 of FIG. 10 or 158 of FIG. 11. It becomes difficult to control the formation of the ferroelectric spontaneous polarization reversal region accurately.
Further, the method for forming a ferroelectric spontaneous polarization reversal by the electrode pattern as shown in FIG. 1 or the insulating mask pattern as shown in FIG. 2 has problems that the fineness and the closeness of a ferroelectric spontaneous polarization reversal region are limited depending on the formation accuracy of a stroke width and/or a spacing of each pattern, and that the production process including to form a ferroelectric spontaneous polarization reversal becomes complicated because extra processes for forming patterns and removing the patterns have to be needed in the production process.
The first object of the present invention is to solve the problems described above and to provide a method for forming a ferroelectric spontaneous polarization reversal homogeneously even if the width of a ferroelectric spontaneous polarization reversal region is over 50 μm, and further, to provide a method for forming a ferroelectric spontaneous polarization reversal that enable the lowering of the intensity of an applied voltage in a ferroelectric spontaneous polarization reversal.
The second object of the present invention is to provide a method for forming a ferroelectric spontaneous polarization reversal where the ferroelectric substrate has convexo-concave structure, such as ridge structure and the like, on its surface and the polarity of the region including one portion of said convex part is reversed with accuracy.
Moreover, the third object of the present invention is to provide a method for forming a ferroelectric spontaneous polarization reversal which could form a polarization reversal region closely, accurately and finely on a ferroelectric substrate, and also to provide the method for forming a ferroelectric spontaneous polarization reversal that is capable of preventing a production process from being complicated.